Gearboxes having coaxial input shafts have been the subject of intensive development efforts in recent times, due to the expectation that in combination with a double clutch for driving the two input shafts they will allow to combine advantages of conventional automatic transmissions such as easy handling and interruption-free shifting, and of conventional manual transmissions such as fast shifting and high power efficiency.
A gearbox having coaxial input shafts is known (e.g., from DE 199 23 185 A1). In this prior art gearbox, first and second layshafts carry first and second pinions which mesh with an output differential. A reverse gear is established by driving a reverse gearwheel on a first one of these layshafts via an intermediate gearwheel on a third layshaft. Since the transmission ratio of the reverse gear should be similar to that of the first or second gears, the reverse gearwheel must be rather large, similar to the driven gearwheels of the first and second gear. Therefore, the gearbox tends to be bulky and difficult to integrate in a compact vehicle design.
This problem is aggravated by the fact that the gearbox requires a double clutch at its input side for selectively driving the two input shafts. Obviously, such a double clutch occupies considerably more space than a simple clutch required for driving a single input shaft.
The shortage of space can be relieved somewhat by using a so called wet clutch system, in which clutch plates are cooled by a liquid coolant. Since in such a system the clutch plates are cooled much more efficiently than in a “dry” clutch system without liquid coolant, more power can be dissipated per unit of clutch plate surface, so that a wet clutch system can be designed much more compactly than a dry clutch system rated for the same maximum torque.
In view of the foregoing, at least one object of the present invention is to provide a gearbox for a motor vehicle which can be associated to a double clutch system and can be designed very compactly. Although the need to have a compact gearbox is much more pronounced in case of a dry clutch system, the invention is applicable with dry and wet clutch systems alike. In addition, other objects, desirable features, and characteristics will become apparent from the subsequent summary and detailed description, and the appended claims, taken in conjunction with the accompanying drawings and this background.